documentation: Slow systems can stall RCU grace periods

If a fast system has a worst-case grace-period duration of (say) ten
seconds, then running the same workload on a system ten times as slow
will get you an RCU CPU stall warning given default stall-warning
timeout settings.  This commit therefore adds this possibility to
stallwarn.txt.
Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/stallwarn.txt

@@ -70,6 +70,12 @@ o	A periodic interrupt whose handler takes longer than the time
 	considerably longer than normal, which can in turn result in
 	RCU CPU stall warnings.
 
+o	Testing a workload on a fast system, tuning the stall-warning
+	timeout down to just barely avoid RCU CPU stall warnings, and then
+	running the same workload with the same stall-warning timeout on a
+	slow system.  Note that thermal throttling and on-demand governors
+	can cause a single system to be sometimes fast and sometimes slow!
+
 o	A hardware or software issue shuts off the scheduler-clock
 	interrupt on a CPU that is not in dyntick-idle mode.  This
 	problem really has happened, and seems to be most likely to